In The Name of Allaah, The Most Merciful, The Bestower of Mercy

[لا إلهَ إلاَّ اللَّه وحْدهُ لاَ شَرِيكَ لهُ – There is no deity worthy of worship except Allaah, alone, without partner].

The one who makes this testimony requires knowledge, truthfulness and must act upon what he has testified to. The scholars say that knowledge of this testimony removes a person from the path of the Nasaaraa- those who act but have no knowledge; acting upon it removes a person from the path of the Yahood- those who have knowledge but do not act; and truthfulness removes a person from the path of the hypocrites- those who claim that they believe but have no certainty.

This testimony [لا إلهَ إلاَّ اللَّه وحْدهُ لاَ شَرِيكَ لهُ – There is no deity worthy of worship except Allaah, alone, without partner] has two pillars- Nafi [Negation] and Ithbaat [Affirmation]. A person cannot be from the people of Tawheed [pure Islamic Montheism] until he fulfils these two pillars. [لا إلهَ  – There is no deity worthy of worship (i.e. Negation)] and [إلاَّ اللَّه – Except Alaah (i.e. Affirmation)]. The negation has to be comprehensive and the affirmation has to be specific – comprehensive negation of the worship that is devoted to all other objects of worship, and specific affirmation that only Allaah has the right to be worshipped.

If a person utters the negation without the affirmation, he is not a person of Tawheed [pure Islamic monotheism]; and if he affirms without uttering the negation, he is not a person of Tawheed [pure Islamic monotheism]. If a person were to negate only by saying [لا إلهَ  – There is no deity worthy of worship], he is an atheist; and if he were to affirm that Allaah has the right to be worshipped without negating that other objects of worship do not have the same right, he is a Mushrik [polytheist]. Therefore, one cannot be a person upon Tawheed [pure Islamic Monotheism] until he testifies that none has the right to be worshipped except Allaah.

And due to the mighty affair of this testimony [لا إلهَ إلاَّ اللَّه  – There is no deity worthy of worship except Allaah], it is emphasised through the statement [وحْدهُ لاَ شَرِيكَ لهُ  – alone, without partner]. The affirmation is emphasised through the statement [وحْدهُ – alone]; and the negation is emphasised through the statement [لاَ شَرِيكَ له – without partner].

Source: An Excerpt from Sharh Kitaab At-Tawheed. Lesson 2. Slightly paraphrased